Liquid-liquid extractors or contactors are known in a variety of configurations and are designed to bring about intimate contact between a relatively dense or "heavy" liquid phase and a relatively low-density or light liquid phase. The two liquids, which are generally mutually insoluble and immiscible, are brought into contact with one another in a plurality of stages in order to effect material transfer between them. For example, refinery or chemical-plant liquids may be extracted with selective solvents designed to pick up one or another component from the mixture for subsequent isolation of this component or purification of the mixture from which the component is removed. The system is used in a wide variety of chemical processes and is of a configuration which is generally independent of the particular solvents or liquid phases to be employed.
It has been proposed heretofore to provide such extractors with a plurality of vertically stacked stages which are traversed by the heavy or relatively high-density phase in a downward direction and by the light or relatively low-density phase in an upward direction. Each stage may be provided with mixers for bringing about intimate contact of the two phases, with separators for separating the two phases from one another and means for inducing the flow of one or the other phase through the particular stage.
In one prior-art system for the continuous extraction of one liquid with another, a plurality of chambers are provided in vertically superposed relationship and are provided with separating compartments of rectangular configuration in horizontal cross-section. The pumps, overflow devices and sight glasses are arranged at juxtaposed end faces of the chambers. The extractor allows control of the layers of each phase and the layer of phase mixture between them with the aid of the overflows so that regardless of the throughput of the device the various surfaces of the phase and phase mixtures remain at the same levels.
The advantage of this system, which uses overflow devices to maintain the levels of the several phases, i.e. the relatively dense, the relatively light and mixed phases, is that the same levels are obtained where the phase ratio is approximately constant. However, this system has the disadvantage that the phase ratio cannot readily be changed and that a gas specific is generally formed on each state above the liquid.
A multistage extractor for liquid-liquid extraction as described has been proposed in which the light liquid is recirculated in each stage with the heavy liquid flowing generally from the top to the bottom and the light liquid traversing the system generally from the bottom to the top. Each of the superimposed stages includes a mixing chamber, a separating chamber and a mixing pump for transferring the liquid from the mixing chamber to the separating chamber of the same stage. Each mixing chamber is disposed above the separating chamber of the same stage but on the level of the separating chamber of the next higher stage and the intakes of the mixing pumps are disposed between passages connecting the upper portion of each mixing chamber to the upper portion of the mixing chamber of the next lower stage, and connecting conduits which communicate between the upper portion of each mixing chamber and the upper portion of the separating chamber of the same stage. Corresponding conduits of two superimposed stages are arranged with mirror symmetry.
An extractor of the latter type has been found to be highly stable under light loads but has the disadvantage that the position or level of the phase boundary is fixed and that with higher loading (i.e. greater throughput) the phase boundary tends to move in an uncontrollable manner.
A liquid-liquid extractor has also been described in which each stage comprises a mixer and a separator and, in each stage, part of one phase can be recirculated through the mixer and the separator. In the latter extractor, the separator of adjacent stages and lateral outlets which are disposed on both sides of the intervening bottom and formed the heavy phase can be collected on the bottom while the light phase can be collected beneath the bottom for separate withdrawal and supply to a mixing pump.
While this system has the advantage that the phase boundary can be adjusted as desired through the full height of each stage, it has the disadvantage that termination of operation of the extractor causes the heavy phases to flow toward the lower stages and the light phases to be displaced upwardly to occupy the space formerly occupied by the heavy phase. Thus, with the extractor in a charged condition but out of operation, the entire volume of the heavy phase is found in the lower stages and the entire volume of the light phase has been displaced to the upper stages. As a consequence, starting up again is a time-consuming process requiring development of new phase boundaries at each stage.